Accompanying miniaturization of electronic equipment, a pitch between electrodes of an electronic component mounted on the electronic equipment has also become narrow in recent years. For example, the pitch between the electrodes has been 100 μm or less. As methods of forming any solder bumps on the electrodes having such a fine pitch, a plating method, a solder-ball-mounting method (see, for example, Patent Document 1), and a solder-paste-printing method have been known. The solder-paste-printing method has been widely used in general because of an advantage in productive efficiency.
In the solder-paste-printing method, a mask member having openings corresponding to portions of a substrate onto which solder paste comprising solder powder and flux as main components is printed has been first positioned on the substrate. By using a squeegee, the solder paste has been filled in each of the openings of the mask member. The solder bumps have been then formed on the electrodes by heating and fusing the solder paste filled in the openings.
In the above-mentioned solder-paste-printing method, since any flux residue remains on the solder and/or a surface of the substrate after the solder paste is reflowed, a washing step for washing the flux residue and removing it is required. However, in the above-mentioned washing step, a processing by any organic solvent or the like is necessary so that this causes any increased environmental impact. Therefore, a method of forming flux-less solder bump, which does not require to wash any flux residue, is desired.
As the method of forming the flux-less solder bump, Patent Document 2 discloses a method of using only solder powder, filling the solder powder into openings of a mask member mounted on a substrate by movement of a squeegee while irradiating reducing free radical gas under vacuum and then, fusing the solder powder to form a solder bump.